Power level indicator



Fell 6, 1940- A. P, .1. BOUDREAU f 2,189,560

POWER LEVEL TNDICATOR Filed April 8. 1937 2 Sheets-Sheet l /QZZLe/y amor /w/ @df A. E Joud 'eau/ Feb 6, 1940. I A. P. J. BOUDREAU 2&89660 YowER LEVEL NDIGATOR Filed April 8, 1937 2 sheets-sheet 2 compensatingresistor' Syvum/vbo@ Patented Feb.' e, i940 uNi'rsnv STATES PATENT.OFFICE Ilm INDICATO! Anthony P. .l1-Boudreau. Garden City, N. Y., as-

, signor to The Wests-n Union TelegraphA com writ Iori. llil. Y.. aeoporation ol New.

r enligne anni s.- im, serial No. uam

4 Ulnlnl. l (0l. 17E-188) This invention relates measuring yapparatusand particularly to apparatus suitable for measuring small alternatingcurrent levels.

l l0 oxide type for converting the' small alternating currents to bemeasured into direct currents adaptable for operating the meter. l Ihave found that the resistance of the rectifier increases withdecreasing current at a fairly 'uniform rate. It

' is obvious that in caiibrating the meter this variable reslstance mustbe considered. Moreover lthe impedance of the apparatus would vary withthe current being measured and Vhence wouldn I result in an error due toa mismatching ofthe 20 impedances of the instrument and the line.

.One object of this invention is to provide means to remedy said defectsby introducing an adjustable compensating rheostat in such a manner andso calibrated in terms of the meter readings that Q5 the sum of theresistance of the rectifier, meter and rheostat shall remain-constant atthe desired value for all values of current in the compensated range.Another object is to correctfor the' error in the meter reading due tothe capacity reactso ance of the rectitlers which causes the meter toread lower than the true reading as the frequency increases and ltoprovide an f instrument which may be used over a range of frequencies.

'I'he invention will be understood from the fol- 35 lowing descriptionin connection with the accompanying. drawings in which- Figure 1 is aschematic circuit diagram illusatingone arrangement embodying theinven-Y on. 4 o Figs. 2 and 3 are diagramsshowing modincations embodying theinvention.

Fig. 4 is an enlarged plan view of the iront face of the meter;

Fig. 5 is a diagram illustrating the manner of 5 calibrating theelements of the apparatus.

5o It is further convenient to refer this power to a standard referencelevel, such as one milliwatt and to calibrate the meter in terms ofpower levels with respect to the reference standard.

An attenuationv network which serves as multi- (is plier for increasingthe range of the instrument' isoonnected in'the input and is designed tomahtain the input impedance approximately constant regardless ofsetting. A transformer T may be` required between the network `and thecopper oxide rectiner I for matching the impedance of 5 therectifier-meter combination to the-impedance of the line.

The adjustable attenuator network 3 consists of a variable seriesresistor branch 5 and a variable shuntresistor branch t. Themiero-ammeter 1 MA is provided with 'an indicator 8 adapted to move overa suitably graduated scale Iii. The meter scale and the attenuationnetwork are preferably calibrated in logarithmic electrical transmissionunits, preferably decibels. for any desired u vload impedance. say 600ohms. The total power one milliwatt.

While it has been the purpose in prior power level meters to so designthe series and shunt branches ofthe adjustable attenuator or multipliernetwork that the total impedance of the netu work and the meter shouldremain constant for all adjustmentsfl have found that the resistance -ofthe rectiner increases with decreasing current.

althoughthe variation is fairly uniform. It is apparent therefore thatthe meter calibration 0 must take this variable resistance into accountand also that vthe impedance presented to the line would vary inaccordance with the current being measured. Even if the first error inthe rectifier could be corrected in the calibration, the second 35defect would result in an'error due to a mismatching of the instrumentand the line. VMy present invention overcomes both of said defects byintroducing an adjustable compensating rheostat i5. which may becalibrated in terms of the di) meter reading. As a result of thiscompensating action, the sum of the resistance of the rectifier, l meterand rheostat remains constant at the desired value for all values ofcurrent inthe coxnpensated range. as

The calibration of the instrument. which requires a simultaneouscalibration of the meter scale and the compensating rheostat, is readilyaccomplished by means of a bridge circuit arrangement as shown in Fig.5. The input level d0 to the precision attenuator I6 is adjusted to apredetermined amount by means of Athe adjustable rheostat IT, and thebridge is then balanced by means of the compensatingrheostat il, in the'decibel meter Db, the other arm resistances being 55 cred at 600 ohms.when a balance is obtained. both the meter scale and the compensatingrheostat dial may be marked for that particular power level. Adjustmentof the precision attenuator will control the level at the Db meter. Itshould be noted that the level delivered to the meter will be 6 db.below that of the precision attenuator output because of the 50% loss ofcurrent in the bridge circuit. The dial of the compensating rheostat i5is thus calibrated to the same scale as the true reading. By calibratingboth meter scale and the dial of the compensating ieoctot at a number offrequencies, the utility he instrument con be extended over a rangereeuencies. I have indicated on the face plate 'he meter shown in Fig. 4the scale markings 121e meter dial It and of the compensating estat i5,calibrated at three appropriately s; frequencies, thereby making theinstrument serviceable for any frequency withinthis w The compensatingrheostat IB may be located at positions with relation to the otherelements ci' the instrument. Thus it may be conat position A, B or C asindicated in Fig. 2 and may be connected in series or in parallel A orany combination of the two which will produce the desired compensation.Again four matched resistances may be employed, one located in each armof the bridge rectifier.

instead of the adjustable compensation rheostat, as shown in Fig. 1, Imay employ a compensetting element having natural characteristicscomplementary to that of the rectifier. A lament of ilne tungsten wiremay be used for this purpose. It may be located in the positionindicated at D in the arrangement illustrated in Fig. which shows adifferent arrangement of the rectiiier elements.

l. have shown and described a preferred form of electrical measuringapparatus embodying my invention and modiiications thereof, but it willbe evident to engineers skilled in this art that various changes andmodications may be made therein within the scope of the appended claims.

l. in o. power-level indicator a. source of alternating electric powerand an indicating ap- 'tue comprising a rectifier receiving current fromsaid source. and having the characteristic! o! decreasing in resistancewith increasing current and of decreasing its direct current output inaccordance with increases in the frequency of the alternating currentsupplied thereto, an electrical measuring instrument receiving directcurvariations of current at one of said frequencies,-

and with the adjustment of the resistance corresponding to the indicatedscale calibration at said frequency, the resistance of said apparatuswill remain substantially constant at a iixed value.

2. The method of calibrating a measuring instrument provided with anadjustable compensating resistance which comprises inserting theinstrument and compensating resistance in one arm of a Wheatstone bridgehaving fixed like relsistances in the other three arms connecting asource of electrical energy to said bridge, causing a known current toflow through said instrument and compensating resistance. balancing thebridge by adjusting the compensating resistance and then simultaneouslymarking the scales of y the instrument and the resistance for saidpredetermined current value.

3. In the method set forth in claim 2, calibrating for diiferentfrequencies, comprising the further step of simultaneously adjusting thecompensating resistance to balance the bridge at the desired frequencieswhile known currents flow through the instrument andsimultaneouslymarking the scales of the instrument and the resistancefor said value of current at said frequencies.

4. Apparatus for measuring alternating current electric power,comprising a rectiner for receiving current to be measured, saidrectiiler being characterized by decreasing resistance with increasingcurrent, an electrical measuring instrument connected to receive directcurrent from said rectifier, said instrument being provided with a scalecalibrated in logarithmic power units, and a compensating variablerheostat connected in series circuit with the rectifier, saidcompensating resistance being calibrated at deiinite power levels tocorrespond with said scale calibrations of the rectifier, so that withthe adjustment of the resistance corresponding to the indicated scalecalibration, the sum of the resistance of the rectiiler, the instrumentand rheostat remain substantially constant for all values of current inthe compensated scale range.

ANTHONY P. J. BOUDREAU.

